LM1801 smoke detector circuit design electronic project

 

This smoke detector electronic project is designed using LM1801 and common electronic parts. Smoke detector circuit diagram below does not use ionization detection, gas or optocouplers it use two photoresistors (LDR's) and an LED.
Diode D1 rectifies voltage, R7 reducing voltage at a level suitable for integrated circuit.
The circuit uses a pair phototranzistors for compensation which removes any resistance caused by changes in temperature or aging effects. This circuit is made of network bridge R1, R4, two LDR's (R12, R13) connected to one of the comparator inputs common point of R4 and R13. The other comparator inputs are connected internally to the point of R1 and R2 voltage divider R12 and / R3. This arrangement provides both LDR's polarization to the same voltage, to achieve accurate tracking.
Physically, LDR s should be located so that smoke particles reflect light from the LED (D2) in R3, leading to lower its resistance. Once comparator detects this voltage drop, the integrated circuit commands Th1 triac (SCR) powering siren putting it into operating mode. Circuit sensitivity is adjusted with P1, the LED D2 should be mounted between the two photoresistors at approximately equal distances.

Circuit Diagram: